1. Field of the Invention
The present invention relates to an exposure system used in an exposure process for manufacturing semiconductor devices, etc. and a semiconductor device manufacturing method that includes a step of adjusting each wafer to the best focus by measuring a focus setting value of an exposure apparatus for each wafer when exposure process is performed on a lot-to-lot basis.
2. Description of the Related Art
In a conventional manufacturing process of semiconductor devices such as semiconductor integrated circuits or the like, an exposure apparatus having a large numerical aperture is used to improve a resolution for a micro-sized element pattern. However, as the numerical aperture increases, the allowable focal depth is suddenly reduced due to the distinct formation of the element pattern. Because of this, in the exposure process, it becomes necessary to perform a high precision focus setting for the exposure apparatus and an exposure within an appropriate focal depth range. For example, a focal depth of 0.60 μm is required when the design rule is 0.15 μm. Therefore, a focus measurement is extremely important for executing a high degree of precision in the focus setting.
There have been various conventional methods for the focus measurement. Generally, either a resist image focus measurement or a spatial image focus measurement is implemented, and the best focus is calculated prior to exposure. The focus measurement is implemented to align the surface of the wafer (substrate) mounted on a wafer stage of the exposure apparatus with the focal plane (image plane) of a projection lens of the exposure apparatus. By implementing the focus measurement, the reticle pattern image for each exposure shot can be correctly formed on the wafer surface when exposing the wafer.
In the resist image focus measurement, exposure is performed in a plurality of states having relatively changed positional relationships in an optical axis direction of an exposure light (hereafter referred to simply as an optical axis direction) to the surface of the wafer mounted on the wafer stage of the exposure apparatus and the focal plane of the projection lens of the exposure apparatus, and then a positional relationship (best focus position) to enable formation of a favorable resist pattern is derived. The resist image focus measurement is executed by using a focus sensor. A known focus sensor is a grazing-incidence type auto-focus measurement system (hereafter referred to as an AF system). FIG. 3 is an outline structural diagram showing a constitution of an AF system.
As shown in FIG. 3, the AF system 50 is constituted from a light source 51, a light transmission slit 52, a light receiving slit 53, and a light receiving unit 54. Such AF system 50 performs grazing-incidence of an image formation luminous flux 55 onto the surface of a wafer 12 mounted on a wafer holder 11 on a wafer stage 15 provided by the exposure apparatus, and then forms a slit image 56 on the surface of the wafer 12. Further, photoelectric detection of the reflected light is achieved through the light receiving slit 53 and the light receiving unit 54. The AF system 50 has a two dimensional multipoint AF sensor as the light receiving unit 54, and detects positions in the optical axis direction at multiple points on the wafer surface. Although only a single AF system is shown in FIG. 3, a plurality of AF systems can be equipped.
The AF system 50 detects a camber and unevenness of an exposure area on the wafer 12 and calculates a correction amount for matching the surface of the exposure area with the best focus position. Here, the correction amount is a movement amount (a focus offset) of the wafer stage 15 in the optical axis direction and a tilting amount (a leveling offset) from the level plane. The wafer stage 15 is driven in accordance with the calculated correction amount. In general, a movement in the optical axis direction of the wafer stage 15 based on the focus offset, i.e. the action for aligning a position in the optical axis direction with the focal plane, is called focusing, and a movement for changing the tilt of the wafer surface based on the leveling offset, i.e. the action for aligning the wafer surface with the level plane, is called leveling.
When deriving the best focus position using the resist image focus measurement, a reticle for use in the focus measurement that includes a focus measurement pattern is first mounted in the exposure apparatus and then the focus measurement pattern is transferred to a resist film applied to the surface of a wafer. At this time, the focus setting value set in the exposure apparatus changes, for example, according to each exposure event for a fixed number of shots. By so doing, the focus measurement patterns that correspond to each focus setting value is transferred to the resist film on the wafer. In this method, a certain period of time is required for manufacturing the wafer for use in the focus measurement because exposures for the focus measurement patterns are performed successively a plurality of times on the wafer with different focus setting values. Because time depends on wafer conditions it cannot be said for sure, but a time of about 5 to 15 minutes is necessary for manufacturing the wafer for use in the focus measurement. Here, the wafer conditions indicate the process conditions such as exposure conditions that are determined according to the type of the resist film and exposure time.
After developing the wafer on which the focus measurement patterns are transferred described above, a dimension, such as a line width, of a resist pattern corresponding to each focus setting value is measured. For example, the line width at the bottom of the resist pattern, i.e. the line width at the lower surface of the resist pattern, is measured by an electron scanning microscope. Moreover, the measurement position of the resist pattern is established according to the process conditions used in an actual production of the semiconductor device. Based on the measurement result, a graph is created with the focus setting values of the exposure apparatus as the horizontal axis and the measured dimensions as the vertical axis. Based on the curve obtained in this way, the best focus can be calculated.
The relationship between the focus setting value and the resist pattern dimension obtained by the above resist image focus measurement is inherent to the exposure apparatus and is normally very stable for time. Therefore, the relationship between the focus setting value and the resist pattern dimension obtained by the resist image focus measurement does not commonly change once set as the constant in the exposure apparatus. However, for example, when a significant change is generated in the state of an optical system of the exposure apparatus such as replacing the wafer stage drive unit or adjusting the projection lens aberration, then the resist image focus measurement will be re-implemented.
Art described in, for instance, Japanese Published Unexamined Patent Application No. H10-254123, can be given as a conventional technology relating to the calculation of the best focus using the resist image focus measurement. This Prior Art Citation discloses the implementation of the focus measurement by using a reticle that provides a plurality of stepped surfaces so that the distances to a wafer having a resist film at the time of being placed on a reticle stage are respectively different. With each stepped surface of the reticle, mask patterns are respectively formed that correspond to the pattern for use in the focus measurement. When exposing the resist film on the wafer by using the reticle, even if the focus setting value of the exposure apparatus is constant, it is possible to obtain an equivalent resist pattern when changing the focus setting value of the exposure apparatus. In other words, the relationship between the focus setting value and the resist pattern dimension can be obtained without changing the focus setting value of the exposure apparatus. With this method, because the focus setting value of the exposure apparatus does not change, the manufacturing time of the wafer for use in the focus measurement can be shortened.
On the other hand, the spatial image focus measurement is implemented to also correct the gradual slippage over time that occurs in the measurement result of the AF system 50 after the result of the resist image focus measurement is set as the constant in the exposure apparatus. The correction of the gradual slippage over time is realized by calibrating of the AF system 50. Regularly implemented calibrating of the AF system 50 for correcting the gradual slippage over time is called a focus calibration. The main cause of the gradual slippage over time is the fluctuation in atmospheric pressure surrounding the environment of the exposure apparatus.
An explanation based on FIG. 4 is given of a procedure of the focus calibration. FIG. 4 is an outline structural diagram showing an exposure apparatus and a focus measurement system for performing the focus calibration attaching thereto. The exposure apparatus irradiates an exposure light 1 to a reticle 3 arranged on a reticle stage 2 and projects an image of pattern 3a on reticle 3 onto a wafer stage 15 through a reduction projection lens 5. In the focus calibration, the exposure light 1 is irradiated to a spatial image mark body 104 that provides a spatial image mark 104a placed on the reticle stage 2. At this time, a spatial image projection plate 106 mounted on the wafer stage 15 is arranged at the image formation position of the reducing projection lens 5. The spatial image projection plate 106 has an opening 106a that corresponds to the image of the spatial image mark 104a and the opening 106a is arranged at the image formation position of the spatial image mark 104a. The light entering into the opening 106a is reflected by a mirror 7 arranged immediately below the opening 106a and enters a light receiving sensor 121. In other words, the image of the spatial image mark 104a is detected by the light receiving sensor 121 as optical intensity.
The optical intensity measurement by the light receiving sensor 121 is implemented respectively in a state in which the wafer stage 15 is arranged in differing positions along the optical axis direction (vertical direction in FIG. 4), in other words a state in which the focus setting value of the exposure apparatus is set to different values. By so doing, the relationship between the focus setting value of the exposure apparatus and the optical intensity of the projection image can be obtained. FIG. 5 is a drawing that shows the relationship between the wafer stage position in the optical axis direction and the optical intensity of the projection image of the spatial image mark. The horizontal axis in FIG. 5 corresponds to the wafer stage position in the optical axis direction and the horizontal axis corresponds to the optical intensity of the projection image. In this case, the position of the wafer stage 15 at maximum contrast of the projection image is the best focus position. In this method, a certain period of time is required for the focus measurement because the position in the optical axis direction of the wafer stage 15 is changed and then the optical intensity of the projection image is measured. A time of, for instance, 30 seconds to 60 seconds is required.
In an exposure process for mass producing semiconductor devices, a predetermined number of wafers (for example 25 wafers) as a lot are to be processed for an improvement of operation efficiency. In this case, the quality of the semiconductor device is affected when a variance in focus of 0.05 μm occurs between wafers belonging to the same lot. This type of focus variance is caused by changes in the focus properties of the projection lens due to such things as a rise in temperature due to irradiation of the exposure light 1 and change in temperature of the environment either within or outside the exposure apparatus. In order to minimize slippage in the best focus position in an exposure apparatus, the focus correction is performed on the projection lens itself.
However, when the focus control amount by the projection lens exceeds a fixed value at the time of the focus correction, the focus variance becomes more pronounced. Here, the focus control amount is an inherent value in the exposure apparatus. To reduce focus variance caused by the focus control of the projection lens, the focus calibration described above must be performed immediately prior to exposure on each wafer. In other words, when the focus calibration is performed on each wafer immediately prior to exposure and the focus control is performed by adjusting the position in the optical axis direction of the wafer stage for each wafer, the focus control amount by the projection lens is reduced and does not exceed the fixed value. As a result, the focus variance occurring in the exposure process for each wafer within the same lot can be reduced.